Seal the Deal is a Forsaken trophy and therefore only Forsaken seals count. The easiest of these most probably being Dredgen.
Edit:
Chronicler is now unobtainable, I’m pretty sure Ascendant Challenge triumph(s) are bugged for Cursebreaker.
This leaves Unbroken (Crucible) or Rivensbane (Last Wish raid)
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As far as I've seen, the triumphs for the Ascendant Challenges are still working fine since I've been doing them weekly to get the corrupted eggs and ahamkara bones that I'm missing for those triumphs. The only issue I'm aware of regarding Cursebreaker is related to the nightfall-specific Pulse Rifle from The Corrupted nightfall strike [Horror's Least] since it is required to complete the Dreaming City collection badge, which is needed for the triumph seal. That, however, is a separate issue.